The present invention generally relates to implantable pacemakers, cardioverters and defibrillators and more particularly to a method and apparatus for testing and detecting capture of the heart in response to a pacing pulse energy, deriving and storing stimulation threshold data, and adjusting pacing pulse energy for energy efficiency.
A cardiac pacemaker implantable pulse generator (IPG) is an electrical device used to supplant some or all of an abnormal heart""s natural pacing function by delivering appropriately timed electrical stimulation signals designed to cause the myocardium of the heart to contract or xe2x80x9cbeatxe2x80x9d, i.e. to xe2x80x9ccapturexe2x80x9d the heart. Stimulation pulses provided by implanted pacemakers usually have well-defined amplitude and pulse width characteristics which can be adjusted by remote programming and telemetry equipment to meet physiologic and device power conservation needs of the particular patient.
The strength (amplitude) and duration (pulse width) of the pacing pulses must be of such an energy magnitude above the stimulation threshold that capture is maintained to prevent serious complications and even death. Yet, it is desirable for these energy magnitudes not to be higher than the stimulation threshold than is needed for a reasonable xe2x80x9csafety marginxe2x80x9d in order to prolong battery life.
As a result of these considerations, a great deal of effort has been expended over many years to develop pacemaker IPGs having the capability of automatically testing the stimulation threshold, i.e. providing an xe2x80x9cauto-capturexe2x80x9d detection function, and resetting the pacing pulse energy to exceed the threshold by the safety margin without the need for clinical or patient intervention. A variety of approaches have been taken as reflected by the extensive listing of earlier patents described in commonly assigned U.S. Pat. No. 5,324,310 issued to Greeninger, et al., 5,320,643 issued to Roline, et al., 5,871,512, issued to Hemming, et al. and 5,861,013 issued to Peck et al., all incorporated herein by reference in their entireties.
In such pacemaker IPGs, capture detection approaches have taken a variety of forms in the attempt to overcome the difficulty in detecting the evoked cardiac response wave shape due to polarization of the pacing electrodes employed to deliver the pacing pulse. Some of the approaches that have been taken include blanking intervals for the sense amplifiers combined with efforts to suppress or attenuate or compensate electronically for the post-delivery electrode polarization signal.
The Peck, et al. and Hemming, et al. patents cited above disclose capture detection mechanisms in which a reference voltage in a capture detection circuit is continuously updated and decreased in value as the sense amplifier tracks the sensed signal provided that dV/dt of the sensed signal is less than zero or substantially less than zero. When or if dV/dt of the sensed signal becomes equal to zero or substantially equal to zero, that reference voltage is held to the minimum value, or xe2x80x9cnegative peak,xe2x80x9d it attained during the period of time when dV/dt of the sensed signal was less negative. When or if dV/dt becomes positive or substantially positive thereafter, the difference between the sensed signal and the minimum value attained and tracked previously is amplified. The term xe2x80x9cnegative peak trackingxe2x80x9d is used to describe the operation of the foregoing circuit and method.
Alternatively, the use of separate xe2x80x9cfar fieldxe2x80x9d EGM amplifiers and electrode systems from those xe2x80x9cnear fieldxe2x80x9d electrode systems used in delivering the pacing pulse has been proposed in a variety of configurations, as exemplified by the above referenced ""310 patent. The use of cardioversion/defibrillation electrodes for capture detection is disclosed in U.S. Pat. No. 5,683,431 issued to Wang, also incorporated herein by reference in its entirety.
The present invention is directed toward providing an improved capture detection mechanism, which can reliably distinguish between capture and non-capture, following a delivered pacing pulse. In the particular embodiment disclosed, cardioversion/defibrillation electrodes are employed for capture detection, as generally suggested in the above-cited Wang patent. In particular, a right ventricular cardioversion electrode and a subcutaneous cardioversion electrode, for example, taking the form of the housing of the associated implantable pacemaker/cardioverter/defibrillator may be employed. While the specific embodiment discussed herein is directed toward implementation of the invention in the context of an implantable pacemaker/cardioverter/defibrillator, as discussed below, the invention is also believed valuable in the context of devices in which other electrodes, including pacing electrodes, are employed to detect capture. The present invention is particularly desirable for use in devices which may employ an electrode set in which one of the electrodes employed to deliver a pacing pulse is also employed for capture detection.
When the invention is practiced in the context of a pacemaker/cardioverter/defibrillator as disclosed herein, it may be usefully employed in conjunction with cardioversion/defibrillation electrode systems employing either xe2x80x9ctrue bipolarxe2x80x9d pacing and sensing or xe2x80x9cintegrated bipolarxe2x80x9d pacing and sensing. In the case of a device employing xe2x80x9ctrue bipolarxe2x80x9d sensing, the cardioversion/defibrillation lead system is provided with a pair of smaller surface area pacing electrodes, dedicated to delivering cardiac pacing pulses and sensing heart depolarizations. In the case of a cardioversion/defibrillation lead system employing an xe2x80x9cintegrated bipolarxe2x80x9d electrode system, a small surface area pacing electrode in conjunction with a large surface area cardioversion/defibrillation electrode are employed for cardiac pacing and sensing.
In cardioversion/defibrillation lead systems employing true bipolar pacing and sensing, residual polarization of cardioversion/defibrillation electrodes following delivery of a pacing pulse is minimal. In such cases, the present invention is adapted to detect capture by simply determining that the sensed signal following the delivered pacing pulse exceeds a defined threshold, preferably for a defined period. In the case of a cardioversion/defibrillation electrode system employing integrated bipolar pacing and sensing, some post-pacing polarization may remain on the cardioversion/defibrillation electrode employed during pacing. The capture detection mechanism of the present invention is also adapted to accurately detect capture in such cases, by means of a self-adjusting sensing threshold.
Following delivery of a pacing pulse, the capture detection mechanism of a preferred embodiment of the present invention checks to determine whether significant polarization remains on one or both of the electrodes employed for capture detection. For example, in the case of a device employing a cardioversion/defibrillation lead system employing integrated bipolar sensing and pacing, the capture detection mechanism of the present invention would first check the signal amplitude between the electrodes employed for capture detection, e.g., the right ventricular and subcutaneous cardioversion/defibrillation electrodes. In this embodiment, if no significant polarization signal level is present shortly following the pacing pulse, the capture detection mechanism simply determines whether the signal amplitude following delivery of the pacing pulse exceeds a defined threshold for a defined time interval. If the a signal level following delivery of the pacing pulse indicates that polarization is present on one or both of the capture detection electrodes, the mechanism of the present invention employs an alternative mechanism for capture detection, optimized to detect capture in the presence of post-pacing pulse polarization.
As described in the above-cited Peck, et al and Hemming, et al. patents, negative peak tracking can be employed to determine whether a cardiac pacing pulse has been successful in capturing the heart, in most cases. However, it has been determined by the inventor that the required reversal of slope required to allow the negative peak detector to function is not necessarily always present, even in the context of a pacing pulse that successfully captures the heart. For this reason, in the presence of electrode polarization, the capture detection mechanism of the preferred embodiment of the present invention instead relies upon a detection threshold that decreases with time to a defined constant threshold level. The decreasing portion of the threshold preferably declines linearly and is defined as a function of the waveform of the composite electrode polarization/heart depolarization polarization signal following delivery of the pacing pulse. The detection mechanism of the present invention defines a line extending from the detected peak of the polarization waveform and the lowest point following the polarization waveform, within a defined maximum time interval, e.g., 30 milliseconds. The slope of this line is then reduced by a scaling factor, e.g., 1.5, to define a variable detection threshold. Signals which persist above the higher of this linearly decreasing detection threshold and the defined constant threshold level for a defined duration threshold, e.g., 10 milliseconds, are considered to be an indication of successful capture of the heart. In the disclosed embodiment, the constant threshold level is fixed , however, in other embodiments this threshold may be variable either by physician programming or by means of an automatic sensing threshold adjustment mechanism as known to the art, for example by means of a mechanism in which the value of the constant threshold is varied as a function of the amplitude of previous sensed depolarization waveforms.
By the mechanism of the present invention, capture can be detected even in the event that the composite polarization/heart depolarization wave form does not display the negative peak required by the capture detection mechanism of the Peck, et al. and Hemming, et al. patents. Testing of the capture detection mechanism of the present invention by the inventors indicates that it provides a highly reliable mechanism of distinguishing between capture and non-capture in conjunction with cardioversion/defibrillation lead systems employing xe2x80x9cintegrated bipolarxe2x80x9d sensing, particularly those employing the right ventricular and subcutaneous cardioversion/defibrillation electrodes for capture detection.